The present invention relates generally to methods of treating industrial waste, including spent foundry waste, and more particularly to a method of treating such waste to reduce the leachability of certain toxic metals and to process those wastes to make them suitable for use as a raw material.
Industrial wastes, such as spent foundry sands and slags, typically contain leachable or extractable metallic contaminants. These wastes are then classified as hazardous or nonhazardous, or toxic/non-toxic, depending on the levels of contaminants indicated by various leach modeling methods, including: 1) Toxicity Characteristic Leaching Procedure (TCLP; EPA method 1311, xe2x80x9cTest Methods for Evaluating Solid Waste, Physical/Chemical Methodsxe2x80x9d SW-846); 2) Synthetic Precipitation Leaching Procedure (SPLP; EPA Method 1312, xe2x80x9cTest Methods for Evaluating Solid Waste, Physical/Chemical Methodsxe2x80x9d SW-846); 3) Multiple Extraction Procedure (MEP; EPA method 1320, xe2x80x9cTest Methods for Evaluating Solid Waste, Physical/Chemical Methodsxe2x80x9d SW-846); and 4) Indiana Leaching Method, Neutral (Indiana NL; 329 Indiana Administrative Code 10-7-4(b)(3)(B)).
When the waste is hazardous or toxic, provisions exist for its disposal at a hazardous waste landfill. Because disposal in a hazardous waste landfill is expensive, hazardous or toxic waste is preferably treated to reduce the levels of contaminants to acceptable levels. The waste can then be disposed of at less expense in a non-hazardous waste landfill.
Although disposal of hazardous or toxic waste made non-hazardous or non-toxic by treatment is less costly than disposal of untreated toxic or hazardous waste, disposal of that waste is still relatively expensive. If the waste could be used in another industrial process, significant savings and environmental benefits could be achieved.
Some wastes have physical characteristics that enable them to be used in other industrial processes. For example, addition of pozzolanic coal ash to portland cement mixtures allows the derived concrete to obtain a much greater compressive and shear strength than concretes derived from portland cements alone. More specifically, addition of pozzolanic fly ash to portland cement is essential in obtaining compressive and shear strengths required for construction of certain articles. Furthermore, several states currently allow the use of foundry wastes as alternate raw materials.
However, many states require the levels of contaminants in foundry sands, slags and other industrial waste to be below certain levels prior to their reuse. Therefore, if they are present above those levels, the leachable and extractable metallic contaminants must first be chemically stabilized before the industrial waste can be utilized in another process.
Typical methods of treating industrial waste include adding to the waste either a phosphate compound in combination with a metal oxide and water or Portland cement (and/or cement kiln dust, quicklime, pozzolanic fly/bottom ash, etc.) and water. These methods result in the waste becoming substantially solid and typically add at least ten percent by weight of other non-useful materials to the waste. Because of the need for subsequent crushing to form aggregate, these methods destroy the useful characteristics of wastes as substitute aggregate. For example, the particle size and shape distribution of foundry sands would be detrimentally altered after being treated by these methods. Moreover, although some prior art methods treat industrial waste with only a phosphate-containing compound, no steps are taken to retain or recover the particle size and shape distribution of the waste so it can be used in other industrial process.
There is therefore a need for an economical method to treat industrial wastes such as foundry sands or slags. Specifically, there is a need for a method of treating industrial wastes that leave their useful particle shapes and size distributions unchanged so the wastes may be recycled as raw materials useful in other industrial processes.
Briefly describing one aspect of the present invention, there is provided a method of treating waste contaminated with leachable metallic contaminants by combining the waste with a sulfite such as calcium sulfite. The previously leachable metal contaminants are then complexed as insoluble metal sulfur oxides, and the material is safe for use as a raw material.
In another aspect of the invention, industrial wastes such as foundry sand or slag containing metallic contaminants is treated by first reducing the size of the sand or slag xe2x80x9cclumpsxe2x80x9d or xe2x80x9cchunksxe2x80x9d until the sand or slag is returned to a particle size that is about the same as the size of the sand or slag particles before it was contaminated. The particles are then combined with a reactant effective for converting the metallic contaminants to a non-leachable form, and the product thus formed is useful as a substitute raw material in the production of, for example, concrete. In a preferred embodiment of this second aspect of the invention, the reactant is comprised of a sulfite such as calcium sulfite.
One object of the present invention is to provide a method for recycling industrial waste, including foundry sands and slags.
Further objects and advantages will be apparent from the following description.